any protozoan of the superclass (sometimes class or subphylum) Sarcodina. These organisms have streaming cytoplasm and use temporary cytoplasmic extensions called pseudopodia in locomotion (called amoeboid movement) and feeding. Sarcodines include the genus amoeba (see amoeba) and pathogenic species, e.g., dysentery-causing Entamoeba histolytica. These protozoans' cells may be spherical or irregular in shape; the pellicle (or envelope) is usually thin and flexible. Sometimes there is an external shell (see foraminiferan) or skeleton (see radiolarian). The cytoplasm, composed of ectoplasm and endoplasm, may contain more than one nucleus. Food, which adheres to the body surface or is trapped by pseudopodia, is digested in food vacuoles.

Sarcodines reproduce sexually by syngamy (fertilization) (fusion of two gametes) and asexually by division or budding. In multinucleate forms, cytoplasmic division with distribution of the nuclei occurs. Some sarcodines have flagella during certain stages of their development; in other groups flagellated and unflagellated generations alternate. Sarcodines may be either solitary or colonial. Although some are parasitic on plants or animals, most sarcodines are free-living, feeding on bacteria, algae, other protozoans, or organic debris. The genera are distinguished by the structure of their pseudopodia. See also pseudopodium.